The present invention relates to a magnetic recording medium or, more particularly, to a magnetic recording medium such as audiotapes, videotapes, floppy discs and the like comprising a nonmagnetic substrate and a magnetic recording layer formed thereon consisting of a matrix phase of a cured resinous composition and fine particles of a ferromagnetic material uniformly dispersed in the matrix as the binder as well as to a novel resin composition suitable as the binder resin.
In the technology of magnetic recording media of the above mentioned type, it is practiced in recent years that the surface of a substrate is coated with a magnetic coating composition prepared by uniformly dispersing ferromagnetic particles and other optional ingredients in a radiation-curable synthetic resin followed by drying and curing of the radiation-curable resin by the irradiation with radiation such as high-energy electron beams to form a magnetic recording layer.
Various types of radiation-curable resins have been developed and practically used for the purpose including, for example, urethane acrylate oligomers obtained by introducing acrylic acid into the molecular chain ends of a urethane oligomer by forming an ester linkage, those obtained by the addition reaction of a (meth)acrylate having an epoxy group or carboxyl group to the hydroxy groups of a polymer such as vinyl chloride-vinyl acetate-vinyl alcohol copolymers and those obtained by the addition reaction of a monovinyl monoisocyanato adduct prepared by the addition of a diisocyanate to a monomeric compound having a hydroxy group and a (meth)acrylic group in an approximately equivalent amount to the hydroxy groups in a vinyl chloride-based copolymer.
These prior art binder resins for a magnetic coating composition have problems and disadvantages in common that dispersion of the ferromagnetic particles in the coating composition is not always as good as desired, that the viscosity of the coating composition is relatively high due to the long-chain branched units bonded to the hydroxy groups and that the film strength of the magnetic coating layer formed from the magnetic coating composition is not always high enough.
Various attempts and proposals have been made heretofore to solve the above described problems in the prior art binder resins for magnetic recording media. For example, Japanese Patent Kokai No. 61-59621 proposes a resin obtained by the reaction of isocyanatoethyl methacrylate with a prepolymer having hydroxy groups. Although the problems relative to the viscosity of the magnetic coating composition and the film strength of the magnetic recording layer could be solved by using this resin as a vehicle of the coating composition, no sufficient improvement can be achieved in the dispersibility of ferromagnetic particles in the coating composition and this deficiency is still increased when a ferromagnetic powder having a decreased particle size is used with an object to obtain a magnetic recording medium suitable for high-density recording.
Further, Japanese Patent Kokai No. 4-67314 proposes a resin which is a reaction product of a vinyl chloride-based copolymer having hydroxy groups and polar groups other than hydroxy in a molecule and a monomeric compound having at least one ethylenically unsaturated double bond and an isocyanato group in a molecule but having no urethane linkages in the molecule. When a magnetic coating composition is prepared by using a vinyl chloride-based copolymeric resin having sulfonic acid groups, phosphoric acid groups or carboxyl groups as the polar groups other than hydroxy according to the disclosure in the above mentioned patent document, however, it is found that the improvement in the dispersibility of the ferromagnetic particles in the coating composition is not always as satisfactory as desired. This is presumably because the isocyanato groups in the above mentioned monomeric compound react not only with the hydroxy groups in the vinyl chloride-based copolymeric resin but also with the polar groups other than the hydroxy to cause a decrease in the improving effect of the copolymeric resin on the dispersibility of the ferromagnetic particles by the polar groups other than the hydroxy groups. Further, it is a very difficult matter to independently control the content of the unreacted hydroxy groups and the content of the unreacted polar groups other than hydroxy, which are introduced with an object to improve the dispersibility and orientation of the ferromagnetic particles, so that high performance of the magnetic coating composition with stability can hardly be ensured.